Industrial hydrogenation reactions are often performed by using finely divided powdered slurry catalysts in stirred-tank reactors. These slurry phase reaction systems are inherently problematic in chemical process safety, operability and productivity. The finely divided, powdered catalysts are often pyrophoric and require extensive operator handling during reactor charging and filtration. By the nature of their heat cycles for start-up and shut-down, slurry systems promote co-product formation which can shorten catalyst life and lower yield to the desired product.
An option to the use of finely divided powder catalysts in stirred reactors has been the use of pelleted catalysts in fixed bed reactors. While this reactor technology does eliminate much of the handling and waste problems, a number of engineering challenges have not permitted the application of fixed bed reactor technology to the hydrogenation of many organic compounds. Controlling the overall temperature rise and temperature gradients in the reaction process has been one problem. A second problem is that in fixed bed packed reactors there is a significant pressure drop due to the high flow rates required for hydrogenation. A third problem is that liquid-gas distribution is problematic thus often leading to poor conversion and localized concentration gradients. A fourth problem is that the product water phase in a two liquid phase system tends to block access of the reactant to the active catalyst sites and thereby decrease the reaction rate or, in the alternative result, inconsistent reaction rates.
Monolith catalysts are an alternative to fixed bed reactors and have a number of advantages over conventional fixed bed reactors. These reactors have low pressure drop which allow them to be operated at higher gas and liquid velocities. These higher velocities of gas and liquids promote high mass transfer and mixing and the parallel channel design of a monolith inhibits the coalesence of the gas in the liquid phase.
The following articles and patents are representative of catalytic processes employing monolith catalysts and processes in chemical reactions including the hydrogenation of nitroaromatics and other organic compounds.
Hatziantoniou, et al. in xe2x80x9cThe Segmented Two-Phase Flow Monolithic Catalyst Reactor. An Alternative for Liquid-Phase Hydrogenationsxe2x80x9d, Ind. Eng. Chem. Fundam., Vol. 23, No.1, 82-88 (1984) discloses the liquid phase hydrogenation of nitrobenzoic acid (NBA) to aminobenzoic acid (ABA) in the presence of a solid palladium monolithic catalyst. The monolithic catalyst consisted of a number of parallel plates separated from each other by corrugated planes forming a system of parallel channels having a cross sectional area of 1 mm2 per channel. The composition of the monolith comprised a mixture of glass, silica, alumina, and minor amounts of other oxides reinforced by asbestos fibers with palladium metal incorporated into the monolith in an amount of 2.5% palladium by weight. The reactor system was operated as a simulated, isothermal batch process. Feed concentrations between 50 and 100 moles /m3 were cycled through the reactor with less than 10% conversion per pass until the final conversion was between 50% and 98%.
Hatziantoniou, et al. in xe2x80x9cMass Transfer and Selectivity in Liquid-Phase Hydrogenation of Nitro Compounds in a Monolithic Catalyst Reactor with Segmented Gas-Liquid Flowxe2x80x9d, Ind. Eng. Chem. Process Des. Dev., Vol. 25, No.4, 964-970 (1986) discloses the isothermal hydrogenation of nitrobenzene and m-nitrotoluene dissolved in ethanol using a monolithic catalyst impregnated with palladium. The authors report that the activity of the catalyst is high and therefore mass-transfer is rate determining. Hydrogenation was carried out at 590 and 980 kPa at temperatures of 73 and 103xc2x0 C. Again, less than 10% conversion per pass was achieved. Ethanol was used as a co-solvent to maintain one homogeneous phase.
U.S. Pat. No. 6,005,143 discloses a process for the adiabatic hydrogenation of dinitrotoluene in a monolith catalyst employing nickel and palladium as the catalytic metals. A single phase dinitrotoluene/water mixture in the absence of solvent is cycled through the monolith catalyst under plug flow conditions for producing toluenediamine.
U.S. Pat. No. 4,743,577 discloses metallic catalysts which are extended as thin surface layers upon a porous, sintered metal substrate for use in hydrogenation and decarbonylation reactions. In forming a monolith, a first active catalytic material, such as palladium, is extended as a thin metallic layer upon a surface of a second metal present in the form of porous, sintered substrate. The resulting catalyst is used for hydrogenation, deoxygenation and other chemical reactions. The monolithic metal catalyst incorporates catalytic materials, such as, palladium, nickel and rhodium, as well as platinum, copper, ruthenium, cobalt and mixtures. Support metals include titanium, zirconium, tungsten, chromium, nickel and alloys.
U.S. Pat. No. 5,250,490 discloses a catalyst made by an electrolysis process for use in a variety of chemical reactions such as hydrogenation, deamination, amination and so forth. The catalyst is comprised of a noble metal deposited, or fixed in place, on a base metal, the base metal being in form of sheets, wire gauze, spiral windings and so forth. The preferred base metal is steel which has a low surface area, e.g., less than 1 square meter per gram of material. Catalytic metals which can be used to form the catalysts include platinum, rhodium, ruthenium, palladium, iridium and the like.
EPO 0 233 642 discloses a process for hydrogenation of organic compounds in the presence of a monolith-supported hydrogenation catalyst. A catalytic metal, e.g., Pd, Pt, Ni, or Cu is deposited or impregnated on or in the monolith support. A variety of organic compounds are suggested as being suited for use and these include olefins, nitroaromatics and fatty oils.
There is a report by Delft University, in Elsevier Science B.V., Preparation of Catalysts VII, p. 175-183 (1998) that discloses carbon coated ceramic monoliths where the carbon serves as a support for catalytic metals. Ceramic monoliths were dipped in furfuryl alcohol based polymers and the polymers allowed to polymerize. After solidification the polymers were carbonized in flowing argon to temperatures of 550xc2x0 C. followed by partial oxidation in 10% O2 in argon at 350xc2x0 C. The carbon coated monolith typically had a surface area of 40-70 m2/gram.
The present invention relates to an improved process for the hydrogenation of an immiscible mixture of an organic reactant in water. The two phase immiscible mixture can result from the generation of water during the hydrogenation reaction itself or, by the addition of water to the reactant prior to contact with the catalyst. The improvement resides in effecting the hydrogenation of a two phase immiscible mixture of organic reactant in water in a monolith catalytic reactor having from 100 to 800 cells per square inch (cpi), and passing the two phase immiscible mixture of organic reactant in water through the reactor at a superficial velocity of from 0.1 to 2 m/second in the absence of a cosolvent for the two phase immiscible mixture. In a preferred embodiment, the hydrogenation is carried out using a monolith support with a polymer network/carbon coating and a transition metal catalyst.
Several advantages are achievable in the process through the use of a monolith support and these include:
an ability to effect liquid phase hydrogenation of organic compounds as an immiscible phase in water and in the absence of a cosolvent;
an ability to obtain high throughput of product through the catalytic unit even though the reaction rate may be less than that using a cosolvent;
an ability to effect hydrogenation reactions at a consistent reaction rate; and,
an ability to hydrogenate organic reactants under liquid phase conditions that permit ease of separation of reactants and byproduct;
The present invention relates to an improved process for the hydrogenation of an immiscible mixture (two phases) of an organic reactant in water. The immiscible mixture can result from the generation of water during the hydrogenation reaction or, if desired, by the addition of water to the reactant prior to contact with the catalyst.
There are numerous categories of organic compounds having functional groups that may be hydrogenated as a two phase mixture. The functional group containing compounds include nitro-organics, acid anhydrides and the reaction products of a ketone or aldehyde with ammonia or a primary or secondary amine. The following are hydrogenation reactions involving these functional groups that co-produce water and can be hydrogenated in a monolith reactor.
Nitro Group Reduction
xe2x80x83RNO2+3H2xe2x86x92RNH2+2H2O
where R is aromatic. Many nitro aromatics are capable of undergoing the hydrogenation reaction described by the process of this invention. Typical nitroaromatics are nitrobenzene, nitrotoluenes, nitroxylenes, nitroanisoles and halogenated nitroaromatics where the halogen is Cl, Br, I, or F.
Anhydride Reduction to Lactone or Ether 
Anhydrides such as maleic anhydride and phthalic anhydride are first hydrogenated to xcex3-butyrolactone and phthalide respectively. The xcex3-butyrolactone can be further reduced to tetrahydrofuran.
Reductive Alkylation or Reductive Amination 
When an aldehyde or a ketone is treated with ammonia or a primary or secondary amine in the presence of hydrogen and a hydrogenation catalyst, reductive alkylation of ammonia or the amine or reductive amination of the carbonyl compound takes place. R and Rxe2x80x2 can be aromatic or aliphatic. Examples of aldehydes and ketones useful in the hydrogenation reactions include formaldehyde, cyclohexanone and methyl isopropyl ketone. Reaction products resulting from the reaction of these aldehydes and ketones with primary and secondary amines include N-methylcyclohexylamine, N-methyldicyclohexylamine, N,N-dimethylcyclohexylamine, N-ethylcyclohexylamine, dicyclohexylamine, N,N-diethylcyclohexylamine, N, N, Nxe2x80x2-trimethylaminoethylethanolamine, N-ethyl-1,2-dimethylpropylamine and N,N,Nxe2x80x2,Nxe2x80x2-tetramethylpropanediamine.
By immiscibility of the reaction system leading to the presence of two phases, it is meant that two liquid phases are present at the operating temperature. The solubility of the organic reactant in water is not only a function of temperature but also a function of the solubility of the reaction product(s) with the organic reactant and with water. In some hydrogenation reaction systems, e.g., the hydrogenation of dinitrotoluene, the dinitrotoluene reactant, the toluenediamine reaction product and water produce essentially one liquid phase at stoichiometric reaction conditions of 60% toluenediamine, 39% water and 1% dinitrotoluene. In the hydrogenation of nitrobenzene, however, the reaction products of nitrobenzene, aniline and water, on the other hand, remain as a two phase system throughout the hydrogenation process. The following solubility data is for aniline in water and nitrobenzene in water at different temperatures.
Monolith catalysts employed in the process described herein consist of an inorganic porous substrate, a metallic substrate, or a modified substrate coated with a catalytic metal. The modification can be a coating derived from a carbon or a heat treated network polymer. Often the monoliths are based upon a honeycomb of long narrow capillary channels, circular, square, rectangular or other geometric shape, whereby gas and liquid are co-currently passed through the channels under a laminar flow regime. The flow of gas and liquid in these confined channels and under these conditions promotes xe2x80x9cTaylorxe2x80x9d flow with bubbles of H2 gas squeezing past the liquid. This capillary action promotes very high initial gas-liquid and liquid-solid mass transfer.
The pressure drop within an effective monolith reactor can range from 2 kPa/m to 200 kPa/m for combined gas/liquid superficial velocities between 0.1 to 2 meters/second for 50% gas holdup in a monolith reactor having 400 cpi (cells per square inch). Typical dimensions for a honeycomb monolith cell wall spacings range from 1 to 10 mm between the plates. Alternatively, the monolith may have from 100 to 800, preferably 200 to 600 cpi. Channels may be square, hexagonal, circular, elliptical, etc. in shape.
Catalytic metals suited for the hydrogenation of water immiscible organics are impregnated or directly coated onto the monolithic substrate, a modified substrate or a washcoat which has been deposited onto the monolith. The catalytic metals include those Group VIb, Group VIIb, Group VIII, and Group Ib metals of the periodic table and conventionally used in hydrogenation reactions. Examples of catalytic metal components include cobalt, Raney or sponge nickel, palladium, platinum, copper, ruthenium, rhenium and so forth. Often a mixture of metals are employed, one example being palladium and nickel. For a monolith catalyst impregnated with a washcoat the composition of catalytic metals is typically identified as a weight percent within the washcoat itself. The washcoat may be applied in an amount of from 1 to 50% of the monolith total weight. Typical catalyst metal loadings, then, range from 0.1 to 25% by weight and preferably from 1 to 10% by weight of the washcoat. The catalytic metals may be incorporated into the monolith in a manner generally recognized by the art. Incipient wetness from a salt solution of the catalytic metal is one example of a method for incorporating a metal catalytic component on the monolith substrate or modified monolith.
The superficial liquid and gas velocities in the monolith channels are maintained to effect a desired conversion, e.g., 1% to 99% per pass. Typically, the superficial velocity through the monolith ranges between 0.1 to 2 meters per second with residence times of from 0.5 to 120 seconds.
Although not intending to be bound by theory, when a monolith is used as a catalyst support, the morphology of the monolith surface is important in order to (a) attach the active metal for hydrogenation and (b) in the case of two immiscible liquid phases selectively adsorb the reactant over the other immiscible phase, water, and the product.
In terms of a catalyst support, particularly a carbon film acting as a support for the metal, eliminating microporosity of the carbon surface is advantageous for fast reaction rates and long catalyst life. Small and medium size pores in the surface tend to lead to catalyst deactivation through pore plugging with high molecular weight co-products. Therefore, the carbon monolith, a carbon coated monolith or a polymer network/carbon coated monolith should have a very low surface area for optimum activity, i.e., a N2 BET of from approximately 1 to 15 m2/gram of total surface area of monolith catalyst.
To achieve a polymer network/carbon coated monolith having low surface area, polymer coating solutions may be applied to the wall surface and heated below traditional carbonization temperatures. Examples of polymer solutions include furfuryl alcohol and furfuryl alcohol with other additives such as pyrrole and polyethylene glycol methyl ether; epoxy resins with amines; epoxy resins with anhydrides; saturated polyester with glycerol or other multifunctional alcohols; oil-modified alkyd saturated polyesters, unsaturated polyesters; polyamides; polyimides; phenol/formaldehyde; urea/formaldehyde; melamine/formaldehyde and others. The above procedure can be modified by using a commercially available oligomer or copolymer of furfuryl alcohol.
Carbonization of the polymer coating is effected at relatively low temperature. Temperatures for carbonization range from 250 to 350xc2x0 C. vs 550-900xc2x0 C. commonly used in the prior art. Because of the lower carbonization temperatures used herein, these networked polymers having polar groups will retain some of their functionality and are more like the polymer than carbon. These functional groups can be coupled through reaction chemistry to anchor homogeneous catalyst, homogeneous chiral catalysts or ligands to the polymeric surface. They also have significantly lower surface areas, as stated, supra.
The hydrogenation is effected at temperatures of 60-180xc2x0 C. The hydrogenation pressure can be up to 1600 psig.
The following examples are intended to represent various embodiments of the invention and are not intended to restrict the scope thereof.
Preparation of Low Surface Area Polymer Network/Carbon Coated Monolith
General Procedure
Coating: A network polymer resin can be made from the polymerization of the appropriate monomers or oligomers. As an example furfuryl alcohol is polymerized with an acid at a controlled temperature to produce a coating solution. The acid can be inorganic (i.e. HNO3, HCl, H2SO4) or organic (i.e. aromatic sulfonic). A dried monolith is then soaked in the coating solution for 2-4 minutes, allowed to drip dry (removal of excess coating solution from the channels) and let dry. The channels are blown clear with air if it is observed that the monolith channels have become visually blocked by the polymer solution. The coated monolith is further dried at 80xc2x0 C. under a N2 purge overnight.
Carbonization: The coated monolith is mounted in a tube furnace and purged with N2 while the heat is increased to 110xc2x0 C. for 30 minutes. The tube is then continued to be heated until the tube surface temperature is 280xc2x0 C. and held at 280xc2x0 C. for 2 hours. The furnace is cooled to 260xc2x0 C. and 5% O2/He is introduced instead of the N2. The tube containing the monolith is heated to 280xc2x0 C. and held there for 40 minutes. The carrier gas is switched back to N2 and the heat is turned off. The monolith is removed after reaching room temperature.
Metal Impregnation: The catalytically active metal is incorporated onto the monolith by incipient wetness techniques, dried at 80xc2x0 C. in an oven overnight with N2 purge and then calcined at a tube surface temperature of 280xc2x0 C. using N2. The modified monolith is now ready to be used as a hydrogenation catalyst. The modified monolith could also be pre-reduced before being used as a catalyst. To be more specific, after the carbonization the amount of metal salt to dissolve or standard metal solution to dilute based on previously determined pore volume is calculated. In a typical example of metal impregnation, a 2xe2x80x3 diameter 400 cpi cordierite monolith 2xe2x80x3 in height is placed in a glass beaker containing approximately 80 ml of active metal solution. Additional solution is added to cover the monolith if necessary. The monolith is soaked for approximately 30 minutes or until no bubbles are seen. The solution is poured from the container, the monolith is removed and excess solution from channels is cleared by a low flow of air. The monolith is set in the hood for approximately 1 hr., and periodically checked to see if channels remain cleared. If channels are not clear, blow through with low flow of air. The monolith is placed in an 80xc2x0 C. oven with N2 purge overnight. After removal of the monolith from the oven, let it cool in a desiccator. The monolith is then heated in a tube furnace at a tube surface temperature of 280xc2x0 C. using N2 for 2 hours.
Hydrogenation Rate Determination in Monolith Screening Reactor
A 2-liter batch autoclave reactor was fitted with a dual-function impeller, oriented above a catalyst holder for the monolith, capable of inducing gas and pumping the gas-liquid dispersion through the catalyst bed. For the reactions studied, the typical combined liquid volume of reagents was 1 liter. The autoclave reactor was equipped with a dip tube to transfer the liquid reaction solution to a recovery cylinder. The portion of the reaction solution which was removed, was diluted and an internal standard added. Gas chromatography was used to perform a quantitative product analysis to calculate selectivity and conversion.
The raw hydrogen pressure data was corrected for compressibility. A hydrogen uptake curve was obtained as a function of reaction time. This curve was used to calculate rate data at various stages of conversion.